lentiviral system: cloning strategy

Hi everyone,
Iím currently thinking in some strategies to clone a cDNA in a lentiviral vector system. Usually when I did this in other common expression mammalian systems (pCDNA 3, for example) I added the Kozak sequence just prior my insert observing the correct frame. However, a friend of mine that uses the lentiviral system said to me this is unnecessary and that he usually amplifies a small part of the 5íUTR (around 50 bp) together with his insert. According to him this is sufficient to get a good expression of the transgene. What do I do? Iím afraid get bad results in gene expression by using my friendís strategyÖ
are there someone expert on this matter??